1. Field of the Invention
This invention relates to a four-cycle engine, and more particularly to a general-purpose four-cycle engine suited for use as a drive source for a grass mower, a lawn mower or the like. More specifically, the present invention relates to a four-cycle engine of the type which comprises a cylinder block which has a cylindrical internal chamber, and has a cylinder head provided at one end of the internal chamber which cylinder head has an intake port for supplying fuel into the internal chamber and an exhaust port for exhausting exhaust gas from the internal chamber, a crankshaft rotatable about its axis for reciprocally moving a piston within the internal chamber, and a cam shaft rotatable about its axis, which is generally parallel to the axis of the crankshaft, for moving an intake valve and an exhaust valve in synchronism with the reciprocal movement of the piston, the intake valve and the exhaust valve opening and closing the intake port and the exhaust port, respectively.
2. Related Art
A four-cycle engine of this type is disclosed, for example, in Japanese Patent Unexamined Publication No. 59-70838. This conventional four-cycle engine is shown in FIGS. 5 to 7. FIG. 5 is a partly-broken, side-elevational view of the conventional four-cycle engine, FIG. 6 is a partly-broken, plan view of the engine of FIG. 5, and FIG. 7 is a partly-broken, front-elevational view of the engine of FIG. 5.
In FIGS. 5 to 7, this conventional four-cycle engine 100 comprises a cylinder block 101 having a cylindrical internal chamber 102. That portion of the cylinder block 101 disposed at one end of the internal chamber 102 constitutes a cylinder head 103. An intake port 104 for supplying fuel into the internal chamber 102, as well as an exhaust port 105 for exhausting exhaust gas from the internal chamber 102, is formed in the cylinder head 103. The engine 100 further comprises a crankshaft 107 rotatable about its axis X for reciprocally moving a piston 106 within the internal chamber 102, and a cam shaft 110 rotatable about its axis U, which is generally parallel to the axis X of the crankshaft 107, for moving an intake valve 108 and an exhaust valve 109 in synchronism with the reciprocal movement of the piston 106, the intake valve 108 and the exhaust valve 109 opening and closing the intake port 104 and the exhaust port 105, respectively.
The cylinder block 101 has an intake passage 111 and an exhaust passage 112. The exhaust passage 112 extends from the exhaust port 105, and is open to an outer side surface 113 of the cylinder block 101. A muffler 114 is mounted on the outer side surface 113, and is connected to the exhaust passage 112. A carburetor 116 is provided adjacent to an outer side surface 115 of the cylinder block 101 facing away from the muffler 114. The carburetor 116 is connected to the intake passage 111 extending from the intake port 104.
The engine 100 is a four-cycle engine of an overhead valve type in which the rotation of the cam shaft 110 is transmitted to the intake and exhaust valves 108 and 109 via respective push rods 117 and 118 and respective rocker arms 117a and 118a. Both of the vertically extending push rods 117 and 118 are disposed away from the center of the cylinder head 103, that is, near one side of the cylinder block 101, and a straight line Z passing through the push rods 117 and 118 is generally parallel to a straight line Y passing through the center 104C of the intake port 104 and the center 105C of the exhaust port 105 which are disposed generally at the central portion of the cylinder head 103. In this engine 100, the intake passage 111 is extended past that portion lying between the push rods 117 and 118.
In this conventional engine 100, the straight line Y passing through the center 104C of the intake port 104 and the center 105C of the exhaust port 105 is generally parallel to the axis X of the crankshaft 107.
A fan 120 is mounted on an upper end portion 119 of the vertically-extending crankshaft 107, and when the crankshaft 107 rotates, the fan 120 causes a cooling air W to flow toward a lower end 121 of the crankshaft 107, thereby cooling the engine 100. Namely, the cooling air W is caused to flow parallel to the axis X of the crankshaft 107 from the upper end to the lower end of the engine body. A through hole 122, formed in that portion of the cylinder block 101 disposed between the intake port 104 and the exhaust port 105, allows the passage of the cooling air W. In this engine 100, the exhaust port 105 is disposed downstream of the intake port 104 with respect to the direction of flow of the cooling air W.
Reference numeral 123 denotes a fuel tank, reference numeral 124 an air cleaner, reference numeral 125 a crankcase, reference numeral 126 a head cover, reference numeral 127 a cowling cover, reference numeral 128 a spark plug, reference numeral 129 a recoil starter, and reference numeral 130 a cam.
In the above conventional engine 100, the straight line Y passing through the center 104C of the intake port 104 and the center 105C of the exhaust port is generally parallel to the axis X of the crankshaft 107. Although it is considered that there are various reasons for this, it is thought that the major reasons are as follows. Namely, in the engine 100, the axis U of the cam shaft 110 is parallel to the axis X of the crankshaft 107, and since the push rods 117 and 118 extend generally vertically in order to minimize the space occupied by through holes for the push rods 117 and 118, the line Z passing through the push rods 117 and 118 driven by the respective cams 130 spaced from each other along the length of the cam shaft 110 is also generally parallel to the axis X of the crankshaft 107. Further, the two rocker arms 117a and 118a usually have the same length because of a common design, and therefore the intake port 104 and the exhaust port 105 are arranged in such a manner that the line Y passing through their centers 104C and 105C is generally parallel to the axis X of the crankshaft 107.
In the conventional engine 100 of this type, however, there is still room for improvement in the following points.
In the engine 100, the muffler 114 and the carburetor 116 are mounted on the opposite left and right outer side surfaces 113 and 115 of the cylinder block 101, respectively, and on the other hand the intake port 104 and the exhaust port 105 are arranged in such a manner that the line Y passing through their centers 104C and 105C is generally parallel to the axis X of the vertical crankshaft 107, and therefore the sum of the length of the intake passage 111, communicating the carburetor 116 with the intake port 104, and the length of the exhaust passage 112 communicating the exhaust port 105 with the muffler 114 is greater than the distance L between the left and right outer side surfaces 113 and 115, and at least one of the intake passage 111 and the exhaust passage 112 must be long to a certain degree. If the intake passage 111 is long, it takes a longer time for fuel to be fed from the carburetor 116 to the combustion chamber (internal chamber) 102 via the intake passage 111, and besides the resistance to the flow of the fuel increases, which affects the starting characteristics of the engine. On the other hand, if the exhaust passage 112 is long, it takes a longer time for the exhaust gas to pass through the exhaust passage 112, and therefore the amount of heat dissipating from the exhaust gas to the engine body increases, and besides the resistance to the flow of the exhaust gas increases. As a result, the performance of the engine is affected. The engine body may need to be cooled, and the engine body may be subjected to a thermal strain.
As best shown in FIG. 5, in the engine 100, the intake portion comprising the intake port 104 and the intake valve 108 is disposed in registry with the exhaust portion, comprising the exhaust port 105 and the exhaust valve 109, in the vertical direction, that is, in the direction of flow of the cooling air W. Therefore, the exhaust portion at the downstream side of the flow of the cooling air W is not sufficiently cooled by the cooling air W. In the engine 100, although the through hole 122 is provided between the two ports, the through hole 122 is open in a direction (i.e., horizontal direction) perpendicular to the direction (i.e., vertical direction) of flow of the cooling air W. Therefore, the amount of the cooling air W passing through the through hole tends to be insufficient, so that the exhaust portion at the downstream side is not sufficiently cooled by the cooling air W.
In the engine 100, since the intake passage 111 is provided between the two push rods 117 and 118, not only the size (the cross-sectional area of the passage) is limited, but also the inner wall structure of the cylinder head 103 is complicated, and also a mold (e.g. a mold for aluminum die casting) for producing the cylinder head 103 is complicated in structure. And besides, because of the above arrangement of the intake passage 111, the distance between the two rocker arms 117a and 118a is large, and therefore the space occupied by the valve mechanism mounted on the end of the cylinder head 103 is large.
Japanese Patent Unexamined Publication No. 59-70838 describes in FIG. 4 a four-cycle engine in which a crankshaft and a cam shaft are arranged not substantially parallel to each other, but perpendicularly to each other. However, such an engine as described in FIG. 4 of Japanese Patent Unexamined Publication No. 59-70838 can not efficiently transmit the power of the crankshaft to the cam shaft, as compared with the type of engine in which a crankshaft and a cam shaft are generally parallel to each other.